Solvent deasphalting is a technique in the petroleum industry to remove a heavy component asphalt from heavy oil, applicable to heavy oil and oil-sand bitumen, and the atmospheric and vacuum residua resulting from the processing of crude oil. The density at 20° C. of these heavy oils is typically greater than 0.934 g/cm3 (API less than 20) or the boiling point is above 350° C. The deasphalted oil after the removal of asphalt is mainly used as the base oil for lubricants or as the feedstock for subsequent processing such as catalytic cracking or hydroprocessing. The asphalt removed can be used for road pavement and construction materials or as fuel.
The solvent used for the deasphalting process for lubricants is normally propane or butane while for catalytic cracking or hydroprocessing feedstock butane or pentane fractions are often used as solvents. The resultant asphalt is mainly used as fuel or the asphalt component for road construction. The existing deasphalting techniques use either a two stage or a three-stage process. In the first stage, the mixture of the solvent and the heavy oil becomes two phases with the light phase being composed of solvent and deasphalted oil (DAO) and the heavy phase being asphalt phase comprising deoiled asphalt and a certain amount of solvent. After discharging from the extractor, the asphalt phase is heated in a heater to a relatively high temperature to flash off most of the solvent and the remaining solvent is further removed by gas stripping, resulting in deoiled asphalt. In the second stage, the DAO phase is heated to close to the critical point of the solvent or supercritical condition to recover most of the solvent. Steam is used to further strip off the remaining solvent to produce DAO. When a three-stage process is applied, DAO is heated to a higher temperature or reduced to a low pressure to lower the dissolving capacity of the solvent so that the heavier fraction of DAO (resin) settles in the second stage separation. The DAO is heated again to a higher temperature or reduced to a lower pressure for third stage recovery of solvent. The resin and the DAO are stripped to further remove the remaining solvents, resulting in the so-called heavy DAO (or resin) and light DAO.
In this traditional three-stage separation process, the quality control of the DAO is achieved by heating the DAO phase to a higher temperature with a heat exchanger so that the resin in the DAO will settle in the second stage of separation. The separation efficiency is only one equilibrium stage and could not achieve good DAO quality with high yield from heavier feedstock. In order to achieve even such performance, the feeding of the separation column for the resin in the three-stage separation process still needs a complicated heat exchange system.
Based on existing solvent deasphalting processes in either two-stage or three-stage methods, the heating of the asphalt phase is a key factor restricting the yield of the DAO. In order to obtain higher yield of DAO, heavier solvents (such as pentane or hexane) are generally used. However, the softening point of the resultant asphalt will also be higher, which means that the asphalt must be heated to a higher temperature to remove solvent. Under such high temperatures (much higher than the softening point), asphalt undergoes chemical decomposition and condensation, which leads to formation of coke and carbonaceous materials. Besides, asphalt of high softening point (greater than 100° C., especially greater than 150° C.) is highly viscous even at high temperatures, which makes it difficult for discharge and transportation. Therefore, the existing solvent deasphalting processes can not meet the requirements of deep separation of heavy oil.
U.S. Pat. No. 3,847,751 discloses a process for separation of the asphalt phase of a high asphalt content feedstock by heating the asphalt phase to 287-371° C. to remove the solvent and then form granules. Therefore, the problem of heating the asphalt phase using a heater is still not effectively solved.
Chinese patent ZL 01141462.6 “A separation process and its equipment for the removal of asphalt with high softening point in petroleum residua.” discloses a method for the separation of asphalt. In this method, the asphalt phase after solvent extraction was sprayed under throtting and rapid expansion to form asphalt particles with a high softening point. The remaining solvent becomes gaseous after expansion and thus separates from the asphalt particles in a low temperature gas-solid separation process. The advantage of this process is that the recovery of the solvent in the asphalt phase does not require the traditional method of heating with a furnace or flash stripping, which involves a high investment, so that the process scheme is simplified and construction investment is reduced. There are two products from the method given by this patent, i.e., deoiled asphalt particles and DAO. However, there are also limitations with this method. On one hand, while the method is capable of separating solvent from asphalt at a low temperature, the result of the dispersion and granulation of asphalt is controlled by the property of the asphalt phase after extraction and the operational conditions of the extraction column and there are no independent operating parameters to control the size of the asphalt particle, which could even affect the operation of the process. On the other hand, this patent has not effectively addressed the issue with “heavier” feedstocks or the adjustment of relatively poor DAO quality. Therefore, there are some constraints with its application.